In order to monitor the behaviour of gearboxes and similar geared mechanical rotating machinery, and in particular to be able to detect damage therein, it has been proposed that signals representative of vibration produced from the gearbox be examined to provide information relating to the gearbox components. Typically, gearboxes have a number of rotating components, each rotating with a different period defined according to the gear ratios. The vibrations to be analysed are likely to arise from all components and, hence, it is not possible to identify a particular component of a vibration signal with a particular rotating component unless a signal representative of the rotational period of that component can be provided. Furthermore, in order to be able to identify features of the vibration signal with corresponding components in the gearbox, it is necessary to angularly correlate the identified feature of the vibration signal with the particular rotational period of the responsible rotating component. For example, a gear having a missing or damaged tooth may repetitively produce an identifiable vibration feature at a specific position of the gear component as it turns, and in order to identify this vibration feature with the responsible gear component it is necessary to have available a signal representative of the period and phase of rotation of the respective component.
The fitting of a tachometer to a rotating shaft is a well known way to produce an electrical pulse every time a predetermined point on the shaft passes a datum position and allows the occurrence of a vibration signal to be related to the angular position of the responsible vibration-inducing feature of the shaft. However, since gearboxes are commonly enclosed and commonly include many shafts rotating at different speeds it is not possible to fit a tachometer to each shaft in the gearbox. Accordingly, it is normal practice to fit a single tachometer to one shaft in the gearbox and to derive tacho signals for the remainder of the shafts from the single detected tacho signal in accordance with the known relationships between the various intermeshing gears within the gearbox.
Hitherto, it has been usual to use a circuit having a phase locked loop frequency multiplier to achieve the desired frequency and phase relationships between the single detected tacho signal and the signals derived therefrom. However, such prior art circuits suffer from the disadvantage of limited operating frequency range, and inadequate response to changes in rotational frequencies. If the shaft speeds change too rapidly, loss of lock is likely to occur resulting in inability to identify a vibrationinducing feature as a function of shaft angle. There is a need, therefore, for an apparatus which can be attached to a single gearbox shaft but allows the derivation of accurate tacho signals with fixed frequencies and phase relationship for other rotating components within the gearbox. This apparatus should be capable of responding to relatively rapid changes in shaft speed over a wide range of speeds. Additionally, it is preferable for such an apparatus to conveniently add onto vibration analysis equipment, or be incorporated within it.